The 31-year-old has come under criticism from England fans throughout his career, something which Johnson cannot understand.
"The 100 caps is fantastic for Ashley and, for me, no-one deserves it more," said Johnson.
"As a player he's been a proper pro for many years in the Premier League and done some great things in the game.
"It was great to see Stevie (Gerrard) do it not so long ago and Ash has been in the set-up for many years and thoroughly deserves it. He has been a key member of any team he has been involved with season after season.
"Does he get the credit he deserves? Not at times, no. He's been in the set-up for such a long time, he's been a key player and consistent for many years.
"I certainly don't feel he deserves some of the stick he sometimes gets. He is very important for England. He started his 99 caps. If that doesn't tell a story I don't know what does."
Johnson added: "Off the pitch he is one of the nicest guys you are going to meet. He is down to earth, up for a laugh and one of the lads, a top bloke."